1. Field of the Invention
The present invention relates to an apparatus for adjusting the initial position of the surface of a melt, which is suitable for use in a monocrystal growing system employing the Czochralski method to adjust the vertical position of the surface of the melt before the growing of a monocrystal.
2. Description of the Prior Art
A cylindrical bar of silicon monocrystal obtained by the Czochralski method is usually subjected to a grinding process before it is sliced into wafers for making of, for instance, integrated circuit devices. In the grinding process, the outer peripheral surface of the silicon monocrystal bar is ground until the predetermined diameter is attained. Although it is naturally desired that the reduction in diameter caused by the grinding should be minimized in terms of economy, this minimization can be difficult to achieve as desired. In particular, a silicon monocrystal bar obtained by the Czochralski method has an increasingly large diameter (e.g., 158 mm) in recent years, and therefore the proportion of the volume of the material removed from the bar becomes inevitably larger than it was before even if the reduction in diameter is the same, resulting in an undesirable increase in production cost. For instance, even if the reduction in diameter is only 1 mm, the volume of the material removed from an unground bar of 600 mm in length and 158 mm in diameter is 298 cm.sup.3. Since there have been strong demands for lowered production costs particularly in recent years, it is considered to be very advantageous to enable a minimization of the reduction in diameter, particularly with respect to a monocrystal bar of the above-described type, which is expecially costly and precious.
The reduction in diameter can be lessened by, for instance, improving the quick-response performance and the stability of control over the crystal diameter. It can also be lessened by improving the level of precision of a device used in the monocrystal growing system to measure the crystal diameter.
The initial value of the depth of a melt within a quartz crucible has hitherto been calculated by measuring the weight of the silicon polycrystal before it is heated and melted, and by using the specific gravity of the melt.
However, since it is often the case with a quartz crucible that, although its outer diameter is precisely determined, its inner diameter tends to be varied by factors during manufacture. Besides, its inner diameter is also affected by certain factors during use: part of the wall of the crucible dissolves into the melt on its inside, enlarging the inner diameter. For these reasons, the depth value calculated in the conventional manner sometimes has errors on the order of .+-.3 mm. These error levels cause undesired deviations of about 0.3 mm from the theoretical value of the crystal-diameter measurement.